Measuring burner performance is key to optimizing the efficiency of your process heating equipment. Here are the most important factors to monitor.
Boiler fuel-to-steam efficiency
This measurement accounts for both combustion and thermal efficiency, radiation and convection losses. The direct method, also known as the input-output method, calculates efficiency by dividing the boiler output (in British thermal units or Btus) by the boiler input (in Btus) and then multiplying by 100. The actual input and output of the boiler are determined through instrumentation, and the resulting data is used in calculations to determine the efficiency value.
Air-fuel ratio
The air-fuel ratio plays a major role in combustion efficiency. Most high-temperature furnaces and boilers operate with 10% to 15% excess air. Maintaining this level avoids excessive carbon monoxide and nitric oxide (NOx) production. The percentage of excess air is determined by measuring the oxygen content in the flue gas and the following calculation:
Percent Excess Air = Percent oxygen measured divided by (20.9 - percent oxygen measured) x 100
For excess air at 15%, the corresponding oxygen content in the flue gas is approximately 3%. Too much excess air reduces the flame temperature and the available heat. Seasonal changes in temperature and barometric pressure also cause excess air to fluctuate by 5% to 10%. When the air-fuel ratio is optimized, the resulting energy savings usually range from 5% to 25% or more.
Flue gas temperature
If the flue gas temperature is high, it indicates that the heat produced by the boiler isn't being used to create steam and is, therefore, being lost. Flue gas temperature directly correlates with efficiency: boiler efficiency increases by 1% for every 40°F reduction in flue gas temperature.
Boiler design
Good boiler design matches boilers (firetube, watertube, cast iron) with the appropriate burner (pre-mix, radiant, recuperative, regenerative). The choice of burner accounts for boiler geometry, heat transfer, peak capacity and so on.
The boiler's efficiency is also dependent on the ratio of the square feet of boiler surface area to the boiler capacity rating (boiler horsepower). A ratio of around 5 square feet per horsepower is desired.
Burner design
Your burner might not be appropriate for your specific application. Burner manufacturers adjust discharge velocity, flame shape, flame radiance, control methods and flame stoichiometry to match the heat transfer characteristics to the specific application. Oversized burners will also affect performance, leading to a reduction in process thermal efficiency.
Burner controls that independently control fuel and air are best (direct digital linkageless type) because they provide more precise control of the firing rate over the entire firing range. Upgrading your burner control systems offers a significant opportunity to reduce energy operating costs, waste and environmental emissions.
Don't forget maintenance
A burner won't operate as designed if not properly maintained. Make sure you have a preventive maintenance program that includes:
- Inspecting burners for wear or damage. Internal components (gas nozzles, mixing plates) should be checked for dirt and debris, wear, excessive oxidation or warping.
- Tuning the burner. If the air/gas ratio is out of adjustment, the burner is operating inefficiently and excess emissions occur. Measuring flue gas temperature, oxygen concentration and carbon monoxide and NOx emissions will help return the ratio to normal.
- Verifying the fuel is burning completely and cleanly. A combustion analyzer will indicate incomplete combustion by detecting aldehydes, high carbon monoxide and unburned hydrocarbons in the exhaust or process.
Always maintain the burner and heating system as recommended by the manufacturer. Maintaining instrumentation is more important for low-NOx burners. If performance remains below par, it may mean it's time to upgrade the burner or air/fuel control system.